(define (sqrt x)
	(define (good-enough? guess x)
		(< (abs (- (square guess) x)) 0.001))

	(define (improve guess x)
	  	(average guess (/ x guess)))

	(define (sqrt-iter guess x)
	  	(if (goods-enough? guess x)
		    	guess
			(sqrt-iter (improve guess x) x)))

	(sqrt-iter 1.0 x))

;;; 词法作用域
(define (sqrt x)
  	(define (good-enough? guess)
	  	(< (abs (- (square guess) x)) 0.001))

	(define (improve guess)
	  	(average guess (/ x guess)))

	(define (sqrt-iter guess)
	  	(if (goods-enough? guess)
		    	guess
			(sqrt-iter (improve guess))))

	(sqrt-iter 1.0))
